Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus that estimates an error of an amount of developer supplied to a developer containing section of a developing device, and determines a replacement time at which a developer bottle for supplying developer to the developing device is to be replaced.
Description of the Related Art
In an image forming apparatus, such as a copy machine and a laser beam printer, bottle near-end determination for determining a replacement time which is immediately before a toner (developer) bottle becomes empty is performed, and when it is determined that the toner bottle is in a near-end state, a message for prompting a user to replace the toner bottle is displayed.
Conventionally, toner is supplied from a toner bottle to a developing device via a toner replenishment container, which is called a hopper. The hopper is provided with a measurement sensor, and a toner replenishment amount error is calculated by using an amount of toner within the hopper, which is detected by the measurement sensor. The toner replenishment amount error is the difference between a nominal toner replenishment amount per one pumping operation and an actual toner replenishment amount. Then, the bottle near-end determination is performed based on a tendency of the toner replenishment amount error to increase in a negative direction as the toner bottle becomes closer to the bottle near-end state.
However, in recent years, image forming apparatuses are required to perform the bottle near-end determination without a measurement sensor to meet a demand for cost reduction of the whole image forming apparatus. Further, in accordance with the size reduction of image forming apparatuses, a configuration is widely employed which directly supplies toner from a toner bottle to a developing device without providing a hopper. An apparatus having the configuration without a hopper is not equipped with a measurement sensor which is provided in the hopper, and hence is required to estimate a toner replenishment amount error without using a toner amount measured by the measurement sensor, and thereby perform the bottle near-end determination.
As the image forming apparatus that estimates the toner replenishment amount error without using the measurement sensor, there has been known an image forming apparatus that uses a pixel count acquired from image data and a toner replenishment amount calculated based on the number of times of toner replenishment. More specifically, there has been proposed a technique of stabilizing toner density by reducing the toner replenishment amount in a case where an amount of change in the cumulative value of the toner replenishment amount with respect to the cumulative value of the pixel count of each formed image is larger than an upper limit value, but increasing the toner replenishment amount in a case where the amount of change is smaller than a lower limit value (see e.g. Japanese Patent Laid-Open Publication No. 2008-20695).
However, although in the above-described technique, the tendency of the toner replenishment amount error can be estimated with respect to a toner bottle as a unit, it is impossible to estimate the toner replenishment amount error which changes in a progressively increasing manner in the negative direction toward the bottle-end state. This causes a problem that it is impossible to properly determine the bottle near-end state of the toner bottle using the estimated toner replenishment amount error.
Further, conventionally, an image forming apparatus of an electrophotographic type forms a toner image based on image data input to the image forming apparatus by consuming toner in developer contained in a developer containing section of a developing device or the like. In such an image forming apparatus, it is known that the density of an image to be formed by the image forming apparatus varies with a ratio of toner in the developer contained in the developer containing section.
For this reason, the conventional image forming apparatuses include one that predicts an amount of toner consumed from the developer containing section (toner consumption amount) through formation of a toner image based on image data, and determines a toner replenishment amount so as to control the ratio of toner in the developer containing section such that it becomes equal to a target value. Note that the toner consumption amount is a theoretically calculated value, and hence there is a small error between an actual consumption amount of toner which is actually consumed from the developer containing section, and the above-mentioned determined amount of toner consumption. That is, even when an amount of toner corresponding to the determined toner consumption amount is supplied, the ratio of toner in the developer containing section sometimes does not necessarily become equal to the target value.
On the other hand, there has been known a toner replenishment device that corrects the toner replenishment amount corresponding to the toner consumption amount, using a correction amount calculated based on the ratio of toner in the developer containing section (see e.g. Japanese Patent Laid-Open Publication No. H04-304486). However, the toner replenishment device described in Japanese Patent Laid-Open Publication No. H04-304486 has a problem that in a case where after a plurality of images each of which consumes a small amount of toner have been formed in a state in which the ratio of toner in the developer containing section is higher than the target value, a plurality of images each of which consumes a large amount of toner are formed, toner is not immediately supplied to the developer containing section.
In the case where a plurality of images each of which consumes a small amount of toner are formed in the state in which the ratio of toner in the developer containing section is higher than the target value, the above-mentioned correction amount takes such a value as will suppress the toner replenishment amount. That is, in the state in which the ratio of toner in the developer containing section is higher than the target value, the above-mentioned correction amount becomes a negative value.
Therefore, when forming images each of which consumes a large amount of toner, after having formed a plurality of images each of which consumes a small amount of toner, the toner replenishment amount calculated based on the toner consumption amount predicted according to the images each of which consumes the large amount of toner and the correction amount becomes equal to or less than 0. As a consequence, even though formation of the images each consuming the large amount of toner is started, so that the ratio of toner in the developer containing section is reduced, toner is not supplied to the developer containing section.